He killed Matt. He killed Pietra.
He'll try again.
I doubt that negotiation is an option, since his backers or colleagues seem to be the cult of Vendemiatrix. So, we need to remove him from the field of play.
a) We need to prepare ourselves so that we can survive another attack better. He'll have undead and possibly mortal backup. They may well be incorporeal.
b) We need to find him and prepare to take the fight to him so that he does not escape.
Dimensional anchor, turn undead, ghost touch on armor and weaponry, see invisible. Things to counteract an absurdly high hide skill. Glitterdust. Poison? Pointy death.
Gather information. If I was him, I'd have bought up a fair quantity of the Ghost touch kit prior to this. He may have left spies to watch and follow people buying GT gear just in case it'll lead us to the new hiding place. Maybe there's a trail we can follow to him. Rachel the elf-ninja is currently unknown to Singh (and thus safe?) and may be connected or useful to go asking questions.
Following an overt attack on us, he may have gone to ground. Possibly he has favourable links with the family that're putting on the party that Shrelkane is going to. Possibly he's got contacts in the necropolis where the vampire lives.
Maybe that vampire would give Singh up if we promised a crusade by the Church in support of their Bishop to go in and get Singh.
See you all on Saturday! (or Friday if you're esran.